Question about Televue refractors

I own a 5 inch Mak and a pair of 15x70 binos. I have been wanting to try other types of scopes hoping to accumulate knowledge that I will use someday to purchase a new scope.

A friend loaned me is 70mm Televue Pronto and I had a chance to use it last night. He only has 2 eyepieces a 50mm wich is about 8X and a 9mm which should be about 45X.

I am very impressed with the quality of the image but for DSO it certainly lacks in power. I can see why this would make a good scope for photography.

Now I know by "rule of thumb" estimations that the maximum useful power on this little sope should be about 150x or so.

My 5 inch Mak should handle 250x but anything beyond 175x starts to be too grainy for my liking, and actually I rarely go above 120x.

My question is how much power can this little Televue scope deliver and still provide sharp clear views with acceptable brightness? ( I realize of course that the quality will go down with any increase in power, but at what point will the sharpness that Televue is known for, give way to a less than satisfactory view in this 70mm version?) Assuming good seeing, of course.

Daryl Crowley

A good refractor on a good night should easily handle the oft quoted 50 to 60x per inch of aperture (roughly 2x per mm). Assuming good optics and good seeing many have maintained good image fidelity at up to 80x to 100x per inch.

However it has also been authoritatively written that eyepieces in combination with telescopes that yield anything smaller that a .5mm exit pupil will not produce an image that has any more detail (this works out to 2x per mm). In essence the image gets bigger, but it degrades. In most cases on most nights, anything smaller than a 1mm exit pupil will not give a better image, just a bigger one.

With the Pronto that would mean that a 6.8mm eyepiece would give you a 1mm exit pupil and a 3.4mm eyepiece would give you a .5mm exit pupil. The respective magnifications would be: about 70x for the 6.8mm and about 140x for the 3.4mm eyepiece. Something like a 2.5mm eyepiece giving 192x would yield impressively larger images of the Moon, Jupiter and Saturn and of course any other bright object you may want to view, but your ability to resolve fine detail will not be improved and will very likely to use your terms "be too grainy".

Barry Simon